For protecting the environment, many novel processes of producing polycarbonate are being developed in place of conventional interfacial polymerization using methylene chloride, and melt transesterification and solid-phase polymerization have been proposed.
Of those, solid-phase polymerization is specifically noted, as being effected at relatively low temperatures to give polycarbonate products of high quality.
It has heretofore been said that the starting polycarbonate prepolymer to be put into a polymerization reactor to be polymerized therein into polycarbonate through solid-phase polymerization is desirably a porous crystalline one having a number-average molecular weight of from 1,000 to 15,000, having a hydroxyl group and an arylcarbonate group at its terminals in a ratio of from 5/95 to 95/5, having a specific surface area of not smaller than 0.2 m.sup.2 /g, and having a degree of crystallinity of not smaller than 5 (see Unexamined Published Japanese Patent Application No. Hei-3-223330).
For producing such a porous and crystalline polycarbonate prepolymer of that type, disclosed is a method comprising introducing a solid or melt of a prepolymer as obtained through prepolymerization, into a crystallizing solvent, followed by grinding it therein under high shear into porous and crystalline prepolymer grains having an average grain size of not larger than 250.mu..
However, solid-phase polymerization using the granular prepolymer obtained in this method is problematic in that the granular prepolymer is partly fused thereby having some negative influences on the polymerization rate and making it difficult to take the polymer product out of the reactor.
Such technical troubles are peculiar to the solid-phase polymerization, and it is desired to clarify the reasons for those troubles and to solve them.